Radio Frequency Identification (RFID) tags have been utilized extensively to trace pallets of merchandise from a point of shipment to a final destination. The tags are typically passive devices that are read out with RF energy, usually in the 900 MHz range. These passive devices are parasitically powered by the RF energy impinging upon the antenna of the tag, thus powering the integrated circuits within the tag, with the result that the tag transmits the identity of the pallet in response to a probing signal from a reader in the vicinity of the tag.
While such RFID tags are now mandated for pallets in some industries, there is increased level of interest in item-level tagging, which involves placing a tag on the item itself as opposed to on a pallet of items.
However, in order to be able to make such tagging strategies possible for low-value items such as toothpaste and the like, techniques are required to be able to manufacture and deposit the tags on items at an overall cost of no more than 5 cents per item or less.
The relatively low price for the tagging of items is not so important in high-value items such as pharmaceuticals, where the tag price may be as much as 25 or 50 cents from start to finish. Rather, mass merchants are interested in keeping track of how much material is on their shelves for inventory control.
This means that, for short ranges, an individual carries a reader with him- or herself and probes the individual items, either in a walk-by scenario or as the items come into the facility, for instance on a conveyor belt. Also envisioned are so-called “smart shelves”, in which the current stock of goods on a merchant's shelves can be remotely monitored and restocked as required.
Note that RFID technology is not merely a bar code technology, but rather one that can store data and, upon request from a reader, output data to a global database. The data can be as simple as a product ID code.
The desideratum using item-level RFID tags is that the whole shipment history of a product from the time it leaves the manufacturing plant to its final destination can be tracked through various hands such as shippers, importers, wholesalers and warehousemen.
If in its simplest embodiment the RFID tag merely contains an identification number, this number is read out along the way during shipment such that the transport history of the item can be ascertained.
It is noted that the current tags are passive tags in that they do not require or have a battery. This is useful because in item-level tagging, low cost is key, there is no space available for batteries and battery shelf life is not a problem.
With respect to tagging of a pallet, it is noted that a pallet is usually placed on a forklift truck and is driven, for instance, into a warehouse where it passes through the warehouse door at which a reader is located. The reader sends out RF energy that charges up the passive tag by transferring energy to the integrated circuits within the tag. The reader then transmits a special code that interrogates the RFID electronics so as to output the tag ID and any other related information stored by the tag.
These passive devices have a range of approximately 30 feet, given the fact that the Federal Communications Commission limits the amount of radiated power from the reader to be 1 watt.
As to the size of the tags that are currently placed on pallets, they are on the order of 2 inches by 2 inches, with the antenna dimensions being the dominating factor. It is noted that the larger the antenna, the greater the range, since a larger tag antenna can capture more energy from a reader. For short-range applications such as monitoring pill bottle inventories, the antenna can be indeed quite small.
Note that with small antennas the amount of energy available for the integrated circuits making up the tag is limited, with the energy being derived from a so-called rectenna that rectifies the RF energy and stores it on a capacitor. In these cases the energy from the capacitor is utilized to power up the circuitry that includes some kind of logic or even a microcomputer as well as a transmitter. Note that once the circuit is powered up the information is transmitted back to the reader.
Using the above tags to identify pallets is commonplace. However, the integrated circuits are relatively expensive, with the integrated circuit tending to be the most expensive part. Secondary to the expense of the integrated circuit itself is the cost involved in building the tag.
If pallets, for instance, contain high value items, a 50- or 75-cent tag may be affordable; however, for item level tags the cost needs to be kept under 5 cents or less.
Moreover, for item-level tags, the output of the transmitter of the RFID tag is in general in the microwatt range due to the small size antenna required. However, with sufficient size reduction there should be a concomitant cost reduction at least of the integrated circuits. If one could make the integrated circuits very, very small, in the tens of micron size, the cost per IC die goes down dramatically. This is because if one can utilize large wafers, one can make millions of individual die per wafer. With processing costs constant and sufficient yields, one can therefore reduce the cost of the tag under 5 cents.
For item-level tags, for instance on individual pill containers, one can arrange to have antennas that are perhaps a quarter of an inch on a side, with a tiny integrated circuit on them. However, even if one could make the micron-sized RFID tags, one is faced with a significant challenge in how to locate an RFID integrated circuit on the associated antenna at its feed point.
In an effort to reduce the cost of the individual chips, manufacturing large numbers of them on a large-size wafer, while theoretically reducing the cost of these chips, the individual chips are extremely hard to test and hard to handle. What is conventionally done now, at least for pallet-level RFID tags, is to use “pick-and-place” machines and size the individual integrated circuits to be at least large enough to enable the pick-and-place operation. Thus, the integrated circuits must be of a size that they can be taken off some kind of dispensing apparatus and physically moved where they can be deposited on and electrically connected to the antenna.
However, pick-and-place machines currently are limited to integrated circuits that are larger than a millimeter on a side.
If one could break through the barrier imposed by pick-and-place machines, for instance utilizing different deposition techniques, then one could garner the cost savings of manufacturing millions of integrated circuits on a single wafer. It would therefore be extremely useful in reducing the overall price of the RFID tag to be able to have integrated circuits as small as a 10th of a millimeter on a side. Manufacturing of such small integrated circuits is possible with standard 90-nanometer integrated circuit technology. Even 65-nanometer technology in high volume applications is now state of the art.
However, just because one can lay down patterns that have 90-nanometer line widths or less, a serious limitation is the ability to be able to scribe and break the individual ICs apart from the die. Note that various scribing, breaking, and sawing techniques have been used in the past to separate out individual integrated circuits.
Using sawing, for example, the saw blade dimensions defines the kerf, which is the material that the saw blade requires in the removal of material. Note that in the applications being discussed herein, the kerf is larger than the desired size of the chips. This results in very inefficient use of a wafer and therefore added cost per die.
With laser scribing, smaller kerfs may be available. However, thermal issues limit this type of scribing technique to chip dimensions that are still larger than desirable.
Chemical etching is another alternative method. However, conventional approaches lead to severe undercutting of the die, again adding to the kerf dimensions. There is, however, a unique chemical etching process that limits undercutting in which microscopic die can be formed utilizing standard CMOS processes.
Assuming that one can actually separate out the microscopic chips, mounting them to an antenna can be accomplished through the use of a shaped die and a specially shaped receiver cavity. In such so-called “self-assembly methods”, these shaped die are squeegeed over in a slurry across a substrate that has receiver cavities that are adapted to uniquely hold the specially-shaped dies.
This type of self-assembly method, illustrated in U.S. Pat. No. 6,864,570 and licensed to Alien Technology, requires a match between the orientation of the die and the receptacle. Thus the specially shaped ICs have to match the corresponding cavities and if they are randomly oriented in the slurry, they will either not enter the cavity or not be appropriately positioned in the cavity. The result is that the reliability of the RFID tags when manufactured in this and other similar processes often results in failure rates of 5 to 10% that are wholly unacceptable.
In order to eliminate those RFID tags that are inoperative, one must test the tag before applying it to a package, which is another time-consuming and costly procedure that may not be totally successful when microscopic integrated circuit-type tags are involved.
What is therefore needed is first a manufacturing technique for manufacturing RFID tags that reduces the cost of the individual integrated circuit by reducing the size of the integrated circuits; and secondly a technique for coupling the integrated circuits to the feed points of antennas in a way that virtually guarantees a 100% yield while at the same time eliminating the use of pick-and-place machines.